Abstract
Synapses are distributed heterogeneously in neural networks. The relationship between the spatial arrangement of synapses and an individual synapse's structural and functional features remains to be elucidated. Here, we examined the influence of the number of adjacent synapses on individual synaptic recycling pool sizes. When measuring the discharge of the styryl dye FM1–43 from electrically stimulated synapses in rat hippocampal tissue cultures, a strong positive correlation between the number of neighbouring synapses and recycling vesicle pool sizes was observed. Accordingly, vesicle-rich synapses were found to preferentially reside next to neighbours with large recycling pool sizes. Although these synapses with large recycling pool sizes were rare, they were densely arranged and thus exhibited a high amount of release per volume. To consolidate these findings, functional terminals were marked by live-cell antibody staining with anti-synaptotagmin-1-cypHer or overexpression of synaptopHluorin. Analysis of synapse distributions in these systems confirmed the results obtained with FM 1–43. Our findings support the idea that clustering of synapses with large recycling pool sizes is a distinct developmental feature of newly formed neural networks and may contribute to functional plasticity.
Highlights
Current research on synaptic transmission seeks to understand the complex regulation of individual synapses within neural networks
A fit with C density function [9] yielded l = 8.32 (Figure S1C) and was again in the range of previous works [9,10]. These results showed that the properties of the preparation used here were comparable to previous reports and validated that synapses with large recycling pool sizes with high release probabilities and diameters can preferentially be found in synapse clusters
We found that synaptic terminals marked by aSyt1-CypHer were heterogeneously distributed in hippocampal neuronal cultures (Figure 3C)
Summary
Current research on synaptic transmission seeks to understand the complex regulation of individual synapses within neural networks. Decreased neuronal activity results in the establishment of new, less clustered synaptic connections [2,3,4,5] and strengthening of inactive synapses [6]. Besides these structural considerations, functional parameters of synapse populations such as spontaneous activity [7], time-course of endocytosis [8], release probability [9,10], analysis of different synaptic vesicle pools [11,12,13] and single synaptic vesicle exocytosis [14,15,16,17] have been examined. The distribution of features such as vesicle pool sizes or release probabilities was typically skewed positively with a predominance of lower values [6,9,10,18,19] and in some cases approximated by a C density function [9,10]
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